Surge arresters are useful in protecting electronic circuitry from extreme, over-rating transient fault currents. These over-rating transient faults may be caused by switching transients or lightning strikes.
Some surge arresters, especially for higher voltage applications, operate by catastrophic failure of the surge arrester. This is not economically or functionally viable for certain applications, such as power transmission.
One solution to the design of surge arresters is the use of Metal Oxide Varistors ("MOV"). These MOVs along with surge arresters utilizing them are currently manufactured by many manufacturers. However, the Raychem Corporation of California has developed a very high quality MOV that extends its usefulness. The following patents assigned to Raychem are representative of the art.
European Patent No. 0,229,464 to Koch et al. (Pub. Jul. 22, 1987) shows a frangible housing for an electrical component reinforced against explosive shattering by wrapping curable sheet material therearound at spaced apart regions. The wrapped material is cured with ultraviolet radiation. This material holds any pieces shattered by over-voltages together.
European Patent No. 0,230,103 to Koch et al. (Pub. Jul. 29, 1987) discloses a surge arrester where circular varistor blocks are stacked for greater voltage applications.
U.S. Pat. No. 5,039,452 to Thompson et al. (8/91) discloses a process for making ZnO Metal Oxide Varistor (MOV) precursor powder. The powder contains smaller particles of the additive metal oxides evenly distributed throughout the larger particles of the primary metal oxide.
PCT Pat. No. WO 91/14304 (GB 91/00405) to Mikli et al. (Pub. Sep. 19, 1991) discloses a surge arrestor that has eight varesistor blocks stacked together with a fiber-optic cable running through the stack to detect component failure.
PCT Pat. No. WO 93/26017 (US 93/05679) to Wiseman et al. (Pub. Dec. 23, 1993) discloses a method of manufacturing a voltage arrester wherein MOV valve elements are stacked along a longitudinal axis, where the MOV valve elements are compressed between conductive end terminals.
Another solution for surge arresters is the Wagon Wheel.TM. technology as implemented by LEA Dynatech of Tampa, Fla., and used in the Lightning Eliminators and Consultants, Inc. (LEC) TVSS products. This technology is based on U.S. Pat. No. 4,875,137 to Rozanski et al. (Oct. 1, 1989). The LEC TVSS products utilize low or medium sized, individually fused Metal Oxide Varistor ("MOV") in parallel. This is in direct contrast to violent, catastrophic failures, characteristic of large block, encapsulated or other less efficient protection circuits.
There are several problems with MOV based surge arresters. One problem as illustrated in several of the above patents is that MOVs may explode when handling excessive over-voltages. Compounding this problem is the problem that when MOVs are in parallel, such as with the Wagon Wheel.TM. technology above, it is possible that the MOVs have different clamping voltage, and thus a larger than expected proportion of the over-voltage or surge current may flow to a single MOV, thusly destroying that part of the parallel circuit. This may cause a chain reaction of similar individual MOV overloads, ultimately destroying the entire parallel circuit. In the case of MOVs stacked in series, such failure will cause the entire surge arrester to fail, instead of just degrade.
Prior technologies use wire based connections to, and between the MOVs to increase the energy handling capability. These wires introduce inductance that slows the reaction time and causes some variation in response time. In addition, these wires make point contact with the MOV face, thus concentrating the surge energy in a very localized area at the wire. This limits the transfer of surge energy between that wire and the MOV; again leading to the major failure mode, burn through at that point, and uneven distribution of the surge energy. This technology will eliminate that risk.